Refining of sulfur-contaminated tar acids



Nov. 24, 1959 D. c .JoNEs ETAL I REFINING OF SULFUR-CONTAMINATED TAR ACIDS Filed July 7, 1958 United States Patent REFINING OF SULFUR-CONTAMINATED TAR ACIDS Donald C. liones and Martin B. Neuworth, Pittsburgh,

Pa., assignors to Consolidation Coal Company, Pittsburgh, Pa., a corporation of Pennsylvania Application July 7, 1958, Serial No. 747,023

6 Claims. (Cl. 2150-609) This invention relates to the art of rening sulfur-contaminated tar acids, and more particularly to the purification of these tar acids by the separation and recovery of sulfur compounds therefrom in a preferred economically desirable form. y

The two principal commercial sources of phenols or tar acids at present are coke-oven tars and petroleum distillates resulting from oil-cracking processes. The conventional method of recovering tar acids from either source is to extract the tar acids with aqueous caustic solution to produce water-soluble tar acid salts. The caustic solutions containing the tar acid salts are commonly referred to as caustic cresylate solutions. The tar acid salts are separated from the source material by decantation andare sprung, i.e., reconverted to tar acids, by reaction with mineral acids or carbon dioxide. Other sources of tar acids include tar from low-temperature carbonization of coal and lignite and from shale oil distillates.

The extraction of the tar acids byv means of aqueous caustic solution is accompanied by the extraction of sulfur compounds that are usually present as thiols, mercaptans and mercaptides, principally as thiophenols and mercaptides thereof, inasmuch as the thiophenols are even stronger acids than the phenols themselves. The quantity of sulfur compounds present in the original source material varies widely, being sometimes as little as one percent by weight of the tar acids and ranging as high as 25 percent and above. Their presence in the extracted tar acids is undesirable for many industrial applications.

Many schemes have been proposed for the removal of the sulfur compounds from the caustic-extracted tar acids. In almost all of these, the principal consideration has been the purification of the tar acids. The sulfur compounds have generally been regarded as disposable contaminants whose recovery is not of economic significance. The more successful of these schemes are based on the relative ease of oxidation of sulfur compounds from thiophenols to disulides. Ordinarily, it is diicult to keep thiophenols free of disuliides, since normal handling will expose them tov air and results in at least partial oxidation. Accordingly, the proposed schemes, in view of this behavior, have resorted to deliberate oxidation with air to completely convert the thiophenols to disuldes. The latter are readily separable from the tar acids because of their insolubility in an aqueous medium.

The general method of oxidation to disuldes has many disadvantages: irst, if the highly alkaline caustic cresylate solution is directly air-blown, oxidation of the sulfur compounds proceeds at a very slow rate, and usually requires the use of oxidation catalysts; secondly, oxidation at this stage results in a loss of a signiicant quantity of the valuable tar acids; thirdly, on subsequent redistillation of the tar acids, reversion of some remaining disulfides to thiophenols occurs, with a` consequent need for further purification; fourthly, the thiophenol content of the tar acids is still not reduced suiiciently for some commercial applications; ifthly, air-blowing converts the sulfur compounds almost exclusively to disuldes. This is disadvantageous where mercaptans are desired as the end products.

The thiophenols and disuldes are becoming increasingly more valuable articles of commerce, useful per se or as precursors for forming compounds useful as rubber accelerators and vulcanizers, peptizers, antioxidants and the like. Hence, it is commercially advantageous to recover the sulfur compounds in desired proportions of thiophenols and disulfides, these proportions being principally determined by prevailing economic considerations. At the same time it is desirable to obtain this recovery of the sulfur compounds while still providing for maximum recovery of the tar acids in a highly puried form.

Accordingly, it is an object of the present invention to provide an improved method for purifying sulfur-containing tar acids.

It is a further object to provide a method for ing puried tar acids in maximal yield.

It is an additionalA object to provide a method for economically recovering sulfur compound from a caustic cresylate solution containing tar acid salts and sulfur compounds. V

It is still an additional object to provide a method for recovering sulfur compounds from a caustic cresylate solution in desired proportions of thiophenols and disuliides.

In accordance with this invention, a caustic cresylate solution containing tar acids (as salts) and sulfur compounds (as thiophenolates) antiV which has a pH of l2 or higher, is partially neutralized to bring the pH of the solution to between about 9.8 and about 11.0. Thereby two phases are formed. These may be separated bydecantation into organic and aqueous phases. Most of the tar acids and a small portion of the thiophenols are present in this rst organic phase. VA major portion of the thiophenols and a relatively small portion of the tar acids, as free acids and as alkali salts', are present in this rst aqueuos phase. The higher the pH used the greater will be the relative amount of sulfur compounds remaining in the aqueous phase. Depending upon the choice of pH, the organic phase will ordinarily containapproximately to 90 percent of the tar acids originally present and from about 10 percent to more than 40 percent of the sulfur compounds originally present. The thiophenols and tar acids in this rst organic vphase are preferably separated by a double-solvent extraction process, as described in U.S. Patent 2,767,220 and also as described in an improved form in copending application, Serial No. 713,624, tiled February 6, 1958, and assigned to the assignee of this invention. As described in U.S. Patent 2,767,220, thiophenols are separated from tar acids by contacting a mixture thereof with aqueous methanol and a low-boiling naphtha fraction having a density less than 0.8 in a continuous countercurrent extraction zone.

The sulfur compounds present in the rst aqueous phase are next oxidized, preferably by air-blowing, to form two second phases. Oxidation proceeds much more rapidly at the lower pH of 9.8-11.0 compared with the rate found when the initial caustic cresylate solution,

obtainat a pH above 12, is air-blown. Furthermore, the presl ence of an oxidation catalyst is usually not required. Most of the sulfur compounds are oxidized by this airblowing step to disuldes. The two phases formed are separated, the disulfides being recoverable from this second organic phase, being insoluble in aqueous solu-.

formed, with substantially all the remaining amounts of tar acids and thiophenols being present in this third organic phase. The third aqueous phase contains principallyvarious inorganic alkali salts such as the carbonate and' bicarbonate. The third organic phase may be conveniently combined with the iirst organic phase for separation of the tar acids and thiophenols by the double-solvent extraction process previously mentioned.

As a specic feature of this invention, in springing the" tar' acids, i.e., liberating them from their salts, by the two-stage acidification of the caustic cresylate solution, it is preferred to use carbon dioxide as acidifying agent for both stages. It is conveniently available, as from flue gases, and is non-corrosive compared with mineral acids such as sulfuric and hydrochloric acids. The' partial neutralization of the caustic cresylate solution is preferably accomplished at a pH of about 10.8, with the substantial neutralization step being preferably performed at a pH of about 9.5.

For a more detailed description of this invention, in accordance with the objects and features thereof, reference should be made to the sole ligure of the drawing which is a diagrammatic flow sheet of the steps involved in practicing a preferred embodiment of this invention.

Referring to the drawing, a caustic cresylate solution 1 containing sulfur compounds is sprung with carbon dioxide to a pH of about 10.8. The cresylate used for thisV study contained about 40 weight percent wet, crude acids or 28-29 percent acids calculated as net, dry sul fur-free acids. The combined weight of thiols, ie., vthiophenols, and disulfides was approximately 23 percent of the Weight of the tar acids calculated on a sulfur-free basis. A phase separation was accomplished by use of a decanter 2, with approximately 90 percent of the origi nal thiophenols, principally as thiophenolates, and 10 to 20 percent of the tar acids, as free acids and salts, being found in an aqueous phase 3. An organic phase 4 contained 80 to 90 percent of the sprung tar acids and approximately l percent of the original thiophenols.

The tar acids in organic phase 4 are separated from the thiophenols by a countercurrent double-solvent extraction in a solvent extraction column 5. In accordance with the process of U.S. Patent 2,767,220, naphtha having ar density less than 0.80 is fed continuously to the bottom of column 5, While an aqueous methanol solution containing about 40 to 90 Weight percent methanol is fed continuously to the top of column 5. Organic phase 4 is fed to the extraction column at a point intermediate the ends thereof. Naphtha with dissolved thiophenols leaves the top of extraction column 5 and passes to a distillation column 6 where the naphtha solvent is removed by distillation. The aqueous methanol containing dissolved tar acids is withdrawn from the bottom of extraction column 5 and passes to a distillation column 7 Where it is removed by distillation.

The thioplienols in aqueous phase 3 are converted to disuliides by blowing air therethrough. A period of approximately ve hours at a temperature between 40 and 80 C. is considered suitable in this regard, although not critical. Two phases form, and phase separation is Ireadily accomplished, by use of a decanter 8, into an organic phase 9 and an aqueous phase 10. The second organic phase 9 consists principally of disuldes. The second aqueous phase 10 contains free tar acids, tar acid salts and some thiophenols.

Carbon dioxide is blown througth aqueous phase 10 to bring the pH thereof to approximately 9.5, thereby substantially neutralizing the caustic cresylate solution and springing the remaining tar acids from their salts. Two phases'again form, and these are separated by means of decanter 11. An organic phase 12 contains the tar acids `and thiophenols. These may be cycled directly to extraction column 5 for separation or combined with the feedstream to extraction column 5 from first organic phase 4. An aqueous phase 13 consists principally of alkali salts such as the alkali carbonate and bicarbonate salts.

A pH range of 9.8 to 11.0 may be used for the rst partial neutralization step, the lower the pH the greater the amount of tar acids and particularly thiophenols present in the organic phase. The eiect of pH on the recovery of tar acids is shown in Table I.

As may be seen, pH has a significant effect on the recovery of tar acids present in the decanted organic phase, the higher the pH the lower the amount of tar acids recovered therein. In the aqueous phase is found both free tar acids and tar acid salts.

TABLE I Tm' acids, weight percent of raw cresylate springing agent HzSOi C 0s G01 CO1 pH 6.9 9.5-9.6 10.58 10.8

In decanted organic phase 27. 8 25. 0 21. 6 19. 7 Dissolved in aqueous phase 0. 7 3. 1 5.0 6. 6 As sodium salt in aqueous phase. 0. 1. 9 2. 2

Total 28. 5 28. 5 28. 5 2& 5

A -similar comparison may also be made for the recovery of sulfur compounds, both thiophenols and disuldes, at various pHs. This is shown in Table Il.

TABLE II As may be seen therein, there is a marked increase in the percent of sulfur compounds present in the decanted organic phase as the pH is deceased. The data from Tables I and Il may 'also be expressed in terms of the material present in the aqueous phase either as dissolved compounds, Table III, or as unsprung alkali salts, Table IV.

TABLE III f Dissolved organics, weight percent of aqueous phase springing agent HaSOi CO4 CO1 C01 pH 6.9 9.5-9.6 10.58 10.8

Tar acids 0.71 1. 27 2. 09 2. S0 Sulfur compounds 0.03 1.48 0.44 0.23 Neutral oil (excluding di-S) 0. 05 o. 04 0. 05 0. 05

Total u. s 2. s 2. 6 3. 1

TABLE IV Unsprung tar acids and thiols, weight percent 0f aqueous phase springing agent CO: CO2 CO, pH 9.5-9.6 10.58 10.8

Tar acids 0. 12 0.78 0.91 Thiols 0.80 2.76 3.22

Total 0. 9 3. 5 4. 1

After iirst partially neutralizing the aqueous caustic cresylate solution, most of the tar acids are recoveredin the organic phase. As may be noted from Tables I and Il, the pH used for this conversion or springing of thetar acids from their salts, while significantly affecting the amount of tar acids dissolved in the organic phase, much more markedly alects the amount of thiophenols dissolved in this phase. Thus a desired proportioning of the sulfur compounds between thiophenols and disulides may be effectively predetermined at this stage. It is important to note that the thiophenols and tar acids present in the organic phase are preferably separated from one another by a double-solvent extraction process, particularly as shown in U.S. Patent 2,767,220 and in our copending application, Serial No. 713,624. Thereby recovery of sulfur compounds is accomplished as thiophenols, while purified tar acids are also obtained. Air-blowing the aqueous phase at the relatively lower pH enables oxidation to proceed far more rapidly than heretofore practiced and also enables recovery, -as disuliides, of any sulfur compounds remaining in this phase. After substantial neutralization, the resultant organic phase is cycled to the double-solvent extraction column, or combined with the first organic phase. Thereby separation and recovery of thiophenols and tar acids are obtained. Thus a process has been provided for rening sulfur-contaminated tar acids to obtain tar acids in a high degree of purity with substantially little iloss thereof, while at the same time the sulfur compounds are recovered with substantially little loss and in a predetermined form considered economically most desirable. This process offers the additional advantage that the final disposable aqueous phase is relatively freer of minor amounts of sulfur compounds than waste liquors produced heretofore by a single-springing operation.

While we have described herein a method for purifying sulfur-contaminated tar acids while recovering sulfur compounds in desired proportions of thiophenols and disulfides, it is readily apparent that modifications may be made therein without departing from the spirit of this invention. For example, while it is preferred and economically desirable to use carbon dioxide for liberation of the tar acids from their salts, other acids may be used for the partial springing operation. Also, for certain purposes it may be desirable to use carbon dioxide for the partial springing operation and sulfuric acid for the final springing step. Thus while we have described this invention with respect to specic preferred embodiments, this invention is not to be limited by the illustrative examples given, but its scope should be determined in accordance with the objects and claims thereof.

We claim:

l. In a method for purifying sulfur-contaminated tar acids yand recovering sulfur compounds from a caustic cresylate solution containing tar acids and a minor porytion of sulfur compounds, the steps of partially neutralizing said cresylate solution to a pH between about 9.8 and about 11.0 whereby rst aqueous and organic phases are formed, separating said first phases, oxidizing sulfur compounds in said first aqueous phase whereby second aqueous and organic phases are formed, separating said 66 second phases, substantially neutralizing said second aqueous phase -to a pH between about 8.5 and about 9.8

whereby third aqueous and organic phases are formed,v

separating said third phases, and recovering sulfur compounds from the organic phases.

2. The method for purifying sulfur-contaminated tar acids while recovering sulfur compounds in desired proportions of thiophenols and disuldes from a caustic cresylate solution containing tar acids and a minor portion of sulfur compounds, comprising partially neutralizing said solution to a pH between about 9.8 and about 11.0 thereby forming rst aqueous and organic phases, separating said phases, recovering the sulfur compounds as thiophenols from said organic phase, oxidizing sulfur compounds in said irst aqueous phase thereby forming second aqueous and organic phases, separating said second phases, recovering sulfur compounds as disuldes from said second organic phase, acidifying said second aqueous phase to a pH between about 8.5 and about 9.8 thereby forming third aqueous and organic phases, separating said third phase and recovering sulfur compounds as thiophenols from said third organic phase.

3. The method according to claim 2 wherein the caustic cresylate solution is neutralized to a pH of about 10.8 and said rst aqueous phase is neutralized to a pH of about 9.5.

4. The method according to claim 2 wherein carbon dioxide is bubbled through said caustic cresylate solution in an amount to partially neutralize said solution to a selected pH value.

5. Themethod `for purifying sulfur-contaminated tar acids while recovering sulfur compounds in desired proportions of thiophenols and disulfldes from a caustic cresylate solution containing tar acids and a minor portion of sulfur compounds principally -as thiophenols and mercaptides thereof, comprising bubbling carbon dioxide through said solution to a pH of about 10.8 thereby forming irst aqueous and organic phases, said aqueous phase containing a major portion of said sulfur compounds and a minor portion of the tar acids, said organic phase containing inversely corresponding portions, separating said phases, double-solvent extracting said organic phase to separate said tar acids and sulfur compounds principally as thiophenols, blowing air through said aqueous phase sufhcient to oxidize sulfur compounds therein, principally to disuldes, thereby for-ming second aqueous and organic phases, separating said second phases, recovering the disuldes from said second organic phase, bubbling carbon dioxide through said second aqueous phase to a pH of about 9.5 thereby forming third aqueous and organic phases, said third organic phase containing substantially all remaining amounts of tar acids and thiophenols, separating said third phases, and double-solvent extracting said third organic phase to separate the tar -acids and thiophenols.

6. The method according to claim 5 wherein said third organic phase is combined with said first organic phase for doub1e-solvent extraction therewith.

yNo references cited. 

1. IN A METHOD FOR PUROFYONG SULFUR-CONTAMINATED TAR ACIDS AND RECOVERING SULFUR COMPOUNDS FROM A CAUSTIC CRESYLATE SOLUTION CONTAINING TAR ACIDS AND A MINOR PORTION OF SULFUR COMPOUNDS, THE STEPS OF PARTIALLY NEUTRALIZING SAID CRESYLATE SOLUTION TO A PH BETWEEN ABOUT 9.8 AND ABOUT 11.0 WHEREBY FIRST AQUEOUS AND ORGANIC PHASES ARE FORMED, SEPARATING SAID FIRST PHASES, OXIDIZING SULFUR COMPOUNDS IN SAID FIRST AQUEOUS PHASE WHEREBY SECOND AQUEOUS AND ORGANIC PAHSES ARE FORMED, SEPARATING SAID SECOND PHASES, SUBSTANTIALLY NEUTRALIZING SAID SECOND AQUEOUS PHASE TO A PH BETWEEN ABOUT 8.5 AND ABOUT 9.8 WHEREBY THIRD AQUEOUS AND ORGANIC PHASES ARE FORMED, SEPARATING SAID THIRD PHASE, AND RECOVERING SULFUR COMPOUNDS FROM THE ORGANIC PHASES. 